Electrical insulator having additional protective insulating portion

ABSTRACT

An electrical insulator having an insulating portion, to which an electrode is applied, whose resistance to leakage currents is greater than the leakage current resistance of the remaining portion of the insulator to prevent the formation of an electrical discharge between the electrode and the remaining portion of the insulator.

United States Patent [191 Pohl [451 May 21, 1974 1 1 ELECTRICAL INSULATOR HAVING ADDITIONAL PROTECTIVE INSULATING PORTION [75] Inventor: Dieter Pohl, Nurnberg, Germany [73] Assignee: Siemens Aktiengesellschaft, Munich,

Germany [22] Filed: Aug. 24, 1972 [2]] Appl. No.: 283,407

[30] Foreign Application Priority Data Aug. 30, 1971 Germany 2143365 [52] US. Cl 174/188, 174/140 R, 174/178, 174/209 [51] Int. Cl. 1101b 17/32, HOlb 17/42 [58] Field of Search 174/137 R, 137 A, 137 B, 174/140 R, 140 C, 140 S, 141 R, 141 C, 176, 177, 178, 179, 195, 209, 210, DIG. l, 180,

1,725,097 8/1929 Naylor 174/209 X 1,764,434 6/1930 Cochran 174/179 2,897,386 7/1959 Jones 174/209 X 2,909,591 10/1959 Baker et a1 174/140 R X 3,098,894 7/1963 Sprigings 174/178 3,192,312 6/1965 Saucr 174/209 3,356,791 12/1967 McGowan 174/140 R X FOREIGN PATENTS OR APPLICATIONS 699,704 12/1930 France 174/179 1,181,803 1/1959 France 174/178 406,419 3/1934 Great Britain 174/178 899,578 6/1962 Great Britain l 174/178 159,773 4/1933 Switzerland 174/140 R Primary Examiner-Laramie E. Askin Attorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin 5 7 ABSTRACT An electrical insulator having an insulating portion, to which an electrode is applied, whose resistance to leakage currents is greater than the leakage current resistance of the remaining portion of the insulator to prevent the formation of an electrical discharge between the electrode and the remaining portion of the insulator.

2 Claims, 12 Drawing Figures PATENTEDHAY 21 1914 3,812.2 84

III/[III] Fig. 7 Fig 8 ELECTRICAL INSULATOR HAVING ADDITIONAL PROTECTIVE INSULATING PORTION BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to means for preventing electrical discharge between the body of an insulator, for example an electrical power line insulator, and the electrode or conductor secured thereto.

2. Description of the Prior Art To an increasing extent in electrotechnology, insulator parts are made of organic synthetic materials, which in the following specification are to be understood to include mixtures of organic and inorganic materials, e.g., casting resin materials of organic casting resins with inorganic fillers. However, insulator parts incorporated in stand-off insulators, suspension insulators, bushings, measuring transformer insulators, and the like, are expected to withstand severe operating conditions when used outdoors for high voltage applications. For these applications, inorganic insulating materials such as porcelain, glass, ceramics, and glass ceramics, are predominantly used, because the employment of organic insulating materials leads to failures through breakdowns due to the phenomena described below and herein referred to as tracking.

Unfavorable climatic conditions, such as are found in outdoor installations of electrical power transmission systems, i.e., humidity, dew, rain, particularly in conjunction with dust, acids and salts, cause the formation of an electrically conductive coating on the insulator parts employed in such systems. When voltage is applied, leakage currents develop in these coatings. The resulting heating can lead to local drying of the conductive coating. Such dry spot so formed is then bridged by an electrical or are discharge, until the distance across the dry spot becomes too large and the arc is interrupted. This phenomenon usually occurs in several places and recurs whenever the coating is reformed. The arcs that occur close to the insulators surface constitute a heavy thermal stress of the insulator parts, particularly in the vicinity of the bases of the arcs. Such arcs are particularly damaging to insulator parts made of organic insulating material or plastics, causing ero sion of the surface thereof. Uniform erosion, such as is found with arcs which generally changelocation continuously over the surface of insulator parts, is considered as harmless.

However, dangerous destruction, which eventually initiates a breakdown across the whole insulator, can occur if the arcs settle at individual points and concentrate there. It has been observed in tests, as well as in practice, that this phenomenon particularly occurs at the metallic electrodes, and that the development of a conductive track and the initiation of the breakdown consequently start from there.

The destructive breakdown is hastened by the fact that the arcs on the free surface of an insulator part, hereinafter called field arcs, connect areas which act as effective advanced electrodes as long as they are moist; the forced drying and remoistening cause the base points of the arcs generally to wander about continuously in the two possible dimensions of the insulators surface. In contrast, the metal electrode proper, which is not dependent on the presenceof moisture, is naturally always effective as a source of electrons, so

that in almost all insulator designs, the boundary line between the electrode and the insulator body forms a preferred geometric location for the base point of arcs. Such arcs hereinafter called border arcs, are therefore when generated situated with one base point at the edge of the electrode, and with the other on the surface of the insulator body. The movement of the base point on the electrode side of a border arc is therefore confined to the boundary line between the electrode and the insulator body, i.e., to only one dimension.

In a large number of cases the geometrical configuration of the electrodes, e.g., in the case of rotational symmetry, causes a higher field concentration at or in the vicinity of the boundary line between the electrode and the insulator body.

In the micro-range of the electrode surface, and particularly at the critical boundary line between the electrode and the insulator body, point effects, which further electric field concentration, are assumed to exist, whether the points e. g., scratches, burrs, or surface corrosion, which cause such field effects were produced in the manufacturing process or through subsequent handling.

This geometrically fixed higher electric field concentration causes more heating, earlier drying of the moisture film on the adjoining surface of the insulator body and therefore preferential and more frequent recurrence of arcs at those points. This deleterious process is further enhanced by the fact that the base point of the arc which is fixed on the electrode side, becomes hot and the work function of the electrons at this point of the electrode is correspondingly reduced.

Insulator bodies at such points are therefore subjected to a concentrated thermal stress by the border arcs, the greatest stress occurring at the base point on the electrode side and its vicinity, as the strongest local affinity exists here, while the base point on the insulation side of such border arcs exhibits higher mobility with increasing distance from the electrode, and therewith less destructive intensity.

Insulator bodies made of organic insulating materials can be thermally so overloaded at these points due to the concentrated stress generated by border arcs, that decomposition products are formed which are themselves electrically conductive, or for instance, become indirectly conductive because they absorb appreciable quantities of moisture due to their porosity. Such points constitute an effective advanced electrode, at which and because of their point effect, border arcs again recur. In this manner, the leakage current track advances further and further toward the other electrode, until SUMMARY OF THE INVENTION It is therefore an object of the invention to prevent in configurations of electrodes of different electric potential which are separated from each other by insulator bodies or parts having insufficient or unreliable resistance to the formation of leakage tracks and arcovers caused by such leakage tracks, particularly in insulators made of organic insulating material, the occurrence of leakage tracks which start at the electrodes and then progressively shorten the insulation path between the electrodes until a breakdown occurs.

According to the invention, an electrical insulator comprises a firstinsulating portion having a predetermined leakage resistance characteristic, and at least one additional insulating portion to which an electrode is to be applied. The additional insulating portion has a superior leakage resistance charactertistic and is so proportioned and arranged with respect to the first insulating portion of the insulator that are or electrical discharge between the first insulating portion and an electrode applied to the additional insulating portion is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic, sectional side view of one embodiment of an electrical insulator constructed according to the invention, wherein the additional portion of the insulator is disposed over part of the main body portion thereof; I

FIG. 2 is a plan view of the electrical insulator illustrated in FIG. 1;

FIG. 3 is a schematic, sectional view of another embodiment of an insulator constructed according to the invention, wherein the additional portion is disposed in a recess in the main body portion;

FIG. 4 is a schematic, sectional view of another embodiment of an insulator constructed according to the invention, wherein the additional portion is disposed over the entire boundary area surface of the main body portion of the insulator;

FIG. 5 is a schematic, sectional view of another embodiment of an insulator constructed according to the invention, wherein the additional portion is annular in shape;

FIG. 6 is a schematic, sectional view of still another embodiment of an insulator constructed according to the invention, wherein the additional portion serves as an intermediate element for fastening the electrode in the insulator body;

FIG. 7 is a schematic, sectional view of another embodiment of an insulator constructed according to the invention, wherein the additional portion comprises a tubular body surrounding the electrode;

FIG. 8 is a schematic, sectional view of another embodiment of an insulator constructed according to the invention, wherein the additional portion is constructed in the form of a section of pipe;

FIG. 9 is a schematic, sectional view of another embodiment of an insulator constructed according to the invention, wherein the additional portion is constructed in the form of a cup disposed over the main body portion of the insulator;

FIG. 10 is a schematic, sectional view of still another embodiment of an insulator constructed according to the invention, wherein the additional portion comprises a coating disposed over an electrode cap;

FIG. 11 is a schematic, sectional view of still another embodiment of an insulator constructed according to the invention, wherein the additional portion comprises a petticoat in a conventional power line insulator; and

FIG. 12 is a schematic diagram of an asymmetrical apparatus constructed according to the invention for testing the leakage resistance of insulating materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the invention, the solution of the above-described problem consists in arranging in the boundary region between the electrodes and the main insulator part or body, and between at least one electrode and the insulating body, at least one protective element of an insulating material having a superior leakage-resistance characteristic, wherein the protective elements are so designed, constructed and arranged that the border arcs starting from the electrodes act directly on the protective elements, in such a manner that the insulator body is not subjected to the border arcs and their thermal effects.

In some circumstances, it may therefore be advisable to arrange protective elements at all electrodes. Thus, the formation of a leakage track leading to a breakdown between the electrodes is prevented by the-protective elements in the critical region, i.e., in the vicinity of the electrodes.

As used herein, leakage-resistant insulating materials or materials having a superior leakage resistance characteristic, mean such materials which, if of sufficient thickness, are not decomposed under the action of arcs caused by leakage, particularly by border arcs, either in a manner that a conductive leakage track is formed, or that they are so eroded or destroyed that the border are affects the organic insulating material of the insulator body.

For the purposes of the invention, leakage-resistant insulating materials may be made from inorganic insulating materials such as, porcelain, earthenware, steatite, ceramics, glass, devitrified glass and enamel. Moreover, organic synthetic materials, of which the entire insulator body part cannot be made because of insufficient mechanical strength or cost, can also be used, provided they are resistant to arcs caused by leakage, or are more leakage-resistant than the insulating material of the main insulator body.

Moreover, in accordance with the invention, the protective elements may be constructed from one or several parts, several parts preferably, if a number of material properties are to be combined in the design in order to compensate for stresses due to different coefiicients of expansion, or to enable ease of assembly by use of simple parts.

In many cases, it is sufficient to make the protective elements only so wide1as to fully absorb the thermal effect of the base point of the border are, i.e., the worst stress, on the electrode side. However, it is also advantageous to make the width of the protective elements larger, so that they cannot be bridged by deposits, and so that the base points of the border arcs on the insulating material side are partially or entirely located on the protective elements. In this manner the electrodes are substantially shielded from the surface of the main insulator body. The dimension of the width of the protective element to be used can be determined by those of skill in the art from, among other factors, the maximum or average length of border arcs.

In one particularly advantageous embodiment of the invention, the protective elements are arranged on the main insulator body part directly adjacent the elec trodes, i.e., at the points where the border arcs occur. In other cases, it is advantageous to design them so that they cover parts of the electrodes and parts of the surface of the main insulator body. The border arcs thus act directly on the protective elements.

It is also advantageous to arrange the protective elements on the main insulator body at some small distance from the electrodes (e.g., see FIG. 5) for one or more of the following reasons: (I) to permit large manufacturing tolerances; (2) because the protective element is installed after the electrode is secured to the main insulator body; (3) because a gap is provided for cementing; or (4) because a collar was provided around the electrode when the main insulator body was cast, to assure that, in case of faulty cementing of the protective element, no leakage current which bypasses the protective element can occur. In this design, if the material which fills the space between the electrode and the protective element is not sufiiciently leakage resistant, any leakage track formed between the electrode and the protective body across the exposed surface of the material therebetween cannot lead to a breakdown across the main insulator body, since its path is limited to the width of the gap and is confined by the protective element. Advantageously, the protective element is in this design disposed only so far from the electrode that a leakage track between the electrode and the protective element shortens the overall insulating path between the electrodes by an insignificant amount.

In further accordance with the invention, protective elements are advantageously provided in such a manner that they cover enough of the exposed surface of the electrode (e.g., See FIGS. 7 and 10), so that border arcs starting from an exposed portion of the electrode act only on the protective element which covers the remaining portion of the electrode.

Moreover, it is advantageous only to partially surround the electrode with one or more protective elements, and to dispose these elements only in those areas where border arcs are likely to be generated. This design has particular applicability to asymmetrical insulator configurations.

Furthermore, it is advantageous in many cases to construct the protective element in the form of a ring. In particular, in respect of insulator bodies, which exhibit substantial rotation-symmetry, e.g. power line insulators, it is advantageous from the manufacturing point of view to impart the same rotation-symmetry to the protective elements, (e.g., See FIG. 11).

In typical installations, it may be sufficient to arrange a protective element at only one electrode, if experience has shown that in such specific insulator configuration, the development of the leakage track starts only from this electrode. However, it is frequently advantageous and safer to install the protective elements at all electrodes of an insulator configuration. Identical or different protective elements may be employed at the respective electrodes of such insulator configurations.

In accordance with the invention, it is advisable to secure the protective elements to the surfaces of the main insulator body arrangement in such a way that no currents or discharges, which bypass the protective elements, can occur. To this end, the protective elements are advantageously cemented on or cemented into recesses provided in the main insulator body. In some cases it is preferable to insert the protective elements into the molds in which the main insulator body or its electrodes are cast. If protective elements made of material that can be cast are used, they can be cast into re cesses formed in the main insulator body. They may also be pre-cast so that when the insulator part is cast, they can be cast in the same mold. It is particularly sim ple and advantageous to make the protective elements by applying a coating of the appropriate material on the electrodes and/or insulator body, by immersion, spraying or painting processes.

In some insulator designs, it is also advantageous to enclose the electrodes within the protective element in such a way that the electrodes do not touch the insulator body (e.g., see FIGS. 6 and 10). In this design, a breakdown bypassing the protective element is prevented.

Furthermore, the invention is advantageously employed in insulator configurations which consist of several parts, e.g., suspension insulators assembled as a chain, and structurally long stand-off insulators assembled from parts, where each part has metallic fittings at the end. According to the invention, a few protective elements which are preferably mounted at the outer electrodes of such chains, are sufficient. If required, however, protective elements according to the invention can also be provided at several or all such fittings. Multiple interruptions of the potential leakage paths formed are thereby achieved. This construction is also advantageous with longer insulator parts which consist of one piece, or are so constructed that the insulation path is not interrupted by metal fittings. In this case, protective elements are distributed over the whole insulation path between the electrodes. In respect of electrical power transmission insulators, protective elements may form parts of the insulalator body, e.g., one or several ribs or petticoats, (e.g., see FIG. 11).

In a chain arrangement of several insulators, individual links of the chain, preferably the outer links according to a further embodiment of the invention, are protective elements for the entire chain, in such a manner that the insulator part of the outer links consists of a material that is leakage-resistant in open air, and the insulator parts of the other links are made of less leakage-resistant material.

Insulation arrangements according to the invention find application particularly where made of organic synthetic materials which are imperiled by the formation of leakage tracks and arc-over. Special advantages are obtained in stand-off insulators, suspension insulators, bushings, measuring transformers and insulator bodies in switching plants. However, those skilled in the art will recognize that their use is not limited to only these applications.

The invention can also be used advantageously in test procedures which were originally provided for the evaluation of the leakage resistance of insulating material, for instance, in procedures as described in ASTM D 2303-64 T, entitled Liquid-Contaminant, Inclined- Plane Tracking and Erosion of Insulating Materials". Insulating apparatus constructed in accordance with the invention may be used in this test to investigate the resistance of the protective element to border arcs, the

resistance of the insulator part to field arcs and also the leakage resistance of the overall arrangement.

Referring to the drawings in general, an electrical insulator constructed in accordance with the invention comprises a first insulating portion 1 having a predeter mined leakage resistance characteristic, and at least one additional insulating portion or protective element 3 to which an electrode 2 is to be applied or secured. The additional insulating portion 3 has a leakage resistance characteristic which is superior to that of the insulating portion 1.

In particular, referring to FIG. 1 which is a sectional view taken along the line 11 in the plan view of FIG. 2, the protective element 3 is a simple washer which is attached on the insulator body 1, which may exhibit rotational symmetry. Washer 3 may be inserted into the insulator body mold prior to the casting of insulator body 1 or may be cemented on after casting. Washer 3 abuts the electrode 2.

Referring to the embodiment of FIG. 3, the protective element 3 is sunk into the insulator body 1 by placing it in the mold thereof or by subsequent cementing into a recess therein provided. In this arrangement, any are formed predominantly moves as a creeping discharge tangentially over the boundary line between the I protective element 3 and the insulator body 1. As a result, the boundary line is stressed as little as possible.

In accordance with the embodiment of FIG. 4, the protective element 3 and the insulator body I have substantially the same diameter. In this arrangement, the arc field strength of any leakage are formed decreases in magnitude to the rim field strength by the time it reaches the outer edge of the protective element 3. The insulator body I is therefore relieved of the higher field strength.

Referring to FIG. 5, protective element 3, in the form of a washer, does not abut the electrode 2. As a result, larger manufacturing tolerances in respect of all the parts are possible. The gap 4 may be filled with cement or adhesive, which may also serve for the compensation of different coefficients of expansion of the electrode 2 and the protective element 3. The cement 4 can also be leakage-resistant and together with the washer 3 form one protective element which consists of several parts. Material 4 may also consist of the same insulating material or casting resin, respectively, as the insulator body 1 and may be made integrally with it without loss of the advantages of the invention.

With reference to FIG. 6, the protective element 3 at the same time serves as an intermediate element for fastening the electrode 2 in the insulator body 1. It is at the same time designed so that the electrode 2 is completely separated from the insulator body 1, so that no leakage current can bypass the protective element 3, through gaps between the insulator body I and the protective element.

With reference to FIG. 7, the protective element 3 is designed as a simple tubular body, which surrounds the electrode 2 over a predetermined length. If border arcs start from the uncovered part of the electrode 2, they hit the protective element 3 and are prevented from moving to the insulator body I. In the embodiment shown in FIG. 8, the protective element 3 is a simple section of pipe.

In the embodiment shown in FIG. 9, a cap-shaped electrode 2 is provided. The protective element 3 is designed in the shape of a cup, which is fitted over the insulator body 1 thereby preventing any contact between the electrode 2 and the insulator body I, completely separating the electrode from the insulator body.

Referring to FIG. 10, a protective element 3 is applied on a cap-shaped electrode 2 in the form of a coating, for instance, by glazing. Bypassing of the protective element 3 by leakage currents is not possible in this arrangement even with a poor joint between the elecnode 2 and the insulator body 1. Border arcs must start from the uncovered part of the electrode 2 and will then encounter the protective element 3.

FIG. 11 shows the use of the present invention in a conventional power line insulator I, in which the protective element 3 duplicates one (and where desired several) ribs or petticoats of the insulator. The transition to the main insulator body I is thereby relieved to a large extent with respect to stress by arcs.

FIG. 12 shows an asymmetrical apparatus arrangement, constructed in accordance with the invention and including an insulator body I in the form of a rectangular plate with an electrode 2 placed on one surface thereof. This arrangement has particular application for testing the leakage resistance of insulating materials by means of samples, e.g., in tests according to or similar to ASTM D 2303-64 T, entitled Liquid- Contaminant, Inclined-Plane Tracking and Erosion of Insulating Materials and similar procedures. In this embodiment of the invention, the electrode 5 is of a predetermined shape from which experience shows, the leakage track starts. The electrode 5 is placed on a protective element 3, which is designed as a small rectangular plate and is embedded in the main'insulator body 1. The arrangement results in suppression of the development of a leakage track leading to arc-over, which is caused in the absence of protective element 3, by the border arcs starting from the electrode 5. It is therefore possible to use this apparatus arrangement to investigate the behavior of protective elements, the be havior of the main insulator body in respect of field arcs, and the behavior of the overall configuration.

What is claimed is:

1. An electrical insulator comprising a first portion having a predetermined leakage resistance characteristic consisting of organic synthetic insulating material forming a main body portion of the insulator, and an additional portion having a leakage resistance characteristic superior to said predetermined characteristic to which an electrode is applied consisting of inorganic insulating material and disposed over at least part of a surface of said first portion which is adjacent the boundary region between said electrode and said first portion, said additional portion comprising a cup shaped body disposed on said first portion, and said electrode comprising a cap disposed on said cupshaped body.

2. An electrical insulator comprising a first portion having a predetermined leakage resistance characteristic consisting of organic synthetic insulating material forming a main body portion of the insulator, and an additional portion having a leakage resistance characteristic superior to said predetermined characteristic applied to an electrode consisting of inorganic insulating material and disposed over at least part of a surface of said first portion which is adjacent the boundary region between said electrode and said first portion, said electrode comprising a cup-shaped body disposed over said first portion, and said additional portion comprising a coating of inorganic insulating material disposed on the surface of said electrode. 

1. An electrical insulator comprising a first portion having a predetermined leakage resistance characteristic consisting of organic synthetic insulating material forming a main body portion of the insulator, and an additional portion having a leakage resistance characteristic superior to said predetermined characteristic to which an electrode is applied consisting of inorganic insulating material and disposed over at least part of a surface of said first portion which is adjacent the boundary region between said electrode and said first portion, said additional portion comprising a cup-shaped body disposed on said first portion, and said electrode comprising a cap disposed on said cup-shaped body.
 2. An electrical insulaTor comprising a first portion having a predetermined leakage resistance characteristic consisting of organic synthetic insulating material forming a main body portion of the insulator, and an additional portion having a leakage resistance characteristic superior to said predetermined characteristic applied to an electrode consisting of inorganic insulating material and disposed over at least part of a surface of said first portion which is adjacent the boundary region between said electrode and said first portion, said electrode comprising a cup-shaped body disposed over said first portion, and said additional portion comprising a coating of inorganic insulating material disposed on the surface of said electrode. 